The present invention relates generally to measurement technology, and more specifically to measurement accomplished by successive approximation analog to digital converter technology.
There exists a great number and variety of applications where it is desired to measure a value, such as resistance, voltage, or current, with a fixed resolution. Analog to digital converters (A/D converters) are often used to measure an actual value, analog in form, and then to convert this measurement to a digital value. Successive approximation A/D converters are often used for such applications and provide the advantage of providing a very fast conversion for applications having low resolution, usually 8 bits or lower.
As mentioned above, a standard successive approximation A/D converter may be used to measure an unknown analog value, such as resistance, voltage, or current, and then convert the measured analog value to a digital value. Typically, the unknown analog value is measured with a resolution that is higher than the required resolution and then an offset value is subtracted from the unknown analog value and the result rounded off to the desired resolution. Suppose that 8 bits of resolution is desired. The measurement must be made with a higher resolution than 8 bits, such as 9 bits or 10 bits of resolution, and the measurement result, after subtraction of an offset value from the unknown analog value, must be rounded off to 8 bits of resolution. For instance, if a value between 2 volts and 4 volts is to be measured with 8 bits of accuracy equal to 2.sup.8 or 256 bits, then 9 bits of resolution, 2.sup.9 or 512 bits, must be used to make the actual measurement. Furthermore, if a value between 4 volts and 6 volts is to be measured with 8 bits of accuracy, then 10 bits of resolution equal to 2.sup.10 or 1024 bits must be used to make the actual measurement. After the measurement has been made with a higher degree of resolution than the desired resolution and after subtraction of an offset value from the unknown analog value, then the result is rounded off to the desired resolution, in this case 8 bits resolution.
This type of conversion requires an A/D converter, such as a standard successive approximation A/D converter, with a resolution and accuracy higher than the required resolution and some logic to perform the subtraction. FIG. 1 shows the operation of a typical multiplying digital to analog converter (DAC) portion, similar to the MC 1408, of a standard successive approximation A/D converter. The DAC 10 represents the digital to analog converter portion of a standard successive approximation A/D converter for an 8 bit conversion resolution and comprises a series of eight current sources I0 through I7 represented by In, a series of switches K0 to K7 corresponding to the current sources, and a resistor R0 connected in parallel across Op Amp, an operational amplifier. Current sources I0 to I7 are binarily weighted current sources; thus for this eight bit example, current source 10 is equal to the total current I divided by 2.sup.8 (or 256) while current source I7 is equal to the total current I divided by 2.sup.1 (or 2). Each current source In is connected to a corresponding switch Kn and the current source is electrically connected to input signal 12 of Op Amp only when its switch Kn is closed or equal to "1" as shown in FIG. 1; if switch Kn is open or equal to "0" then current source In is not connected to input signal 12 of Op Amp. In addition to input signal 12, Op Amp has a second input signal 14 which is tied to ground potential as shown.
The Op Amp and resistor R0 in parallel with Op Amp operate to effect a current to voltage conversion such that Output signal 16 of Op Amp is an output voltage. Output voltage signal 16 is set by the digital byte that controls the switches, K0-K7, such that the Output voltage signal 16, V0, is given by the equation: EQU V0=R0*.SIGMA.In*Kn, (1)
where In is the current source selected equal to I/2.sup.(8-n) where I is the total current generated by DAC 10, and Kn is its corresponding switch, Kn, which is either open ("0") or closed ("1").
When the DAC of FIG. 1 is used with a standard successive approximation register, like the MC14549, each switch is closed in sequence, starting with the most significant bit (MSB), and a voltage comparator will leave the switch closed if the resulting output voltage is less than the voltage being measured. On each subsequent cycle the next lower significant bit is closed and the cycle repeats until all 8 (or however many bits of resolution are used) have been completed. The resultant value in the successive approximation register is then the digital representation of the unknown voltage being measured.
While the above implementation of a DAC portion of a standard successive approximation A/D converter does serve to convert a raw analog measurement to a corresponding raw digital value, it does not produce a digital value that is offset with respect to a given measurement range. It is desirable to have a digital value after the conversion is complete that contains an offset, so that a digital value of 0 indicates not a value of 0 Ohms, 0 volts, or 0 Amps, but rather the minimum value of a given range of values. In the above example of measuring voltage between 2 volts and 4 volts, for example, it would be desirable to have a converted digital value of 0 which is representative of the bottom of the measurement range or 2 volts. This may be expressed in equation form where the desired conversion value is represented by: EQU k(X.sub.unknown -X.sub.offset) (2)
where k is a constant, X.sub.unknown is the unknown analog value being measured, and X.sub.offset is the offset value.